The present invention relates to a switching power supply circuit suitable for use in various video apparatus such as a color television receiver and a projector apparatus.
Some video apparatus such as a television receiver and a projector apparatus have an analog circuit and a digital circuit, for example, as circuit blocks for carrying out various signal processing.
Such video apparatus having the analog and digital circuit blocks are provided with a constant-voltage power supply for supplying constant operating voltage to the circuit blocks.
As an example of a conventional power supply circuit provided in such a video apparatus, FIG. 8 shows configuration of a switching power supply circuit provided in a large-sized color television receiver, for example.
A bridge rectifier circuit Di and a smoothing capacitor Ci in the power supply circuit generate a rectified and smoothed voltage Ei corresponding to an alternating input voltage VAC from a commercial alternating-current power.
A self-excited voltage resonance type converter that includes a switching device Q1 and performs switching operation by a so-called single-ended system is provided as a switching converter for interrupting the rectified and smoothed voltage Ei inputted thereto.
The switching device Q1 is driven by a selfoscillation driving circuit formed by a series connection circuit of a driving winding NB, a resonant capacitor CB, and a base current limiting resistance RB. Switching frequency of the switching device Q1 is determined by resonance frequency of a resonant circuit formed by the driving winding NB and the resonant capacitor CB.
A starting resistance RS is provided to supply the switching device Q1 with a starting current obtained in a rectifying and smoothing line at the turn-on of the commercial alternating-current power.
The switching device Q1 is connected with a clamp diode DD1 and a primary-side parallel resonant capacitor Cr shown in FIG. 8. Capacitance of the primary-side parallel resonant capacitor Cr and leakage inductance L1 of the primary winding N1 side of an isolating converter transformer PIT form a primary-side parallel resonant circuit of the voltage resonance type converter.
An orthogonal type control transformer PRT-1 is a saturable reactor provided with a resonance current detecting winding ND, the driving winding NB, and a control winding NC1. The orthogonal type control transformer PRT-1 is provided to drive the switching device Q1 and effect control for constant voltage.
The isolating converter transformer PIT (Power Isolation Transformer) transmits switching output of the switching device Q1 to the secondary side of the switching power supply circuit.
As shown in FIG. 8, a secondary-side winding is formed on the secondary side of the isolating converter transformer PIT by winding secondary windings N2, N3, N4, and N5.
In this case, as shown in FIG. 8, a point of connection between the secondary winding N4 and the secondary winding N5 is connected to a secondary-side ground. A secondary-side parallel resonant capacitor C2 is connected between the secondary-side ground and an ending point of the secondary winding N2 in parallel with the secondary-side winding.
The parallel resonant circuit to convert switching operation into voltage resonance type operation is provided on the primary side of the isolating converter transformer PIT, and the voltage resonant circuit to provide voltage resonance operation is provided on the secondary side of the isolating converter transformer PIT. In the present specification, the switching converter provided with such resonant circuits on the primary side and the secondary side is referred to as a xe2x80x9ccomplex resonance type switching converter.xe2x80x9d
The secondary winding connected in parallel with the secondary-side parallel resonant capacitor C2 is provided with a half-wave rectifying and smoothing circuit formed by a rectifier diode D01 and a smoothing capacitor C01, so that a direct-current output voltage E01 of 135 V for horizontal deflection is obtained from the half-wave rectifying and smoothing circuit.
Also, the secondary winding formed by the secondary windings N3 and N4 is provided with a half-wave rectifying and smoothing circuit formed by a rectifier diode D02 and a smoothing capacitor C02, so that a direct-current output voltage E02 of 15 V for vertical deflection is obtained from the half-wave rectifying and smoothing circuit. The secondary winding N5 is connected with a rectifier diode D03 and a smoothing capacitor C03 shown in FIG. 8, so that a direct-current output voltage E03 of xe2x88x9215 V for the same vertical deflection is obtained from a half-wave rectifying and smoothing circuit formed by the rectifier diode D03 and the smoothing capacitor C03.
Thus, the direct-current output voltages E02 and E03 (xc2x115 V) for vertical deflection are obtained from voltages induced in the secondary winding (N3+N4) and the secondary winding N5 on the secondary side of the isolating converter transformer PIT. Hence, the secondary winding (N3+N4) and the secondary winding N5 have the same number of turns.
In this case, the secondary-side direct-current output voltage E01 is also inputted from a branch point to a control circuit 1.
The control circuit 1 uses the direct-current output voltage E02 as its operating voltage. The control circuit 1 variably controls the inductance LB of the driving winding NB wound in the orthogonal type control transformer PRT-1 by changing the level of a control current flowing through the control winding NC1 according to change in the level of the direct-current output voltage E01. This results in a change in resonance conditions of the resonant circuit including the inductance LB of the driving winding NB in the self-oscillation driving circuit. This represents an operation of changing the switching frequency of the switching device Q1 . This operation makes constant the direct-current output voltages outputted from the secondary side of the isolating converter transformer PIT.
Even with such a configuration for constant-voltage control including the orthogonal type control transformer PRT-1, since the primary-side switching converter is of the voltage resonance type, it may be considered that the power supply circuit variably controls the switching frequency of the switching device Q1 and at the same time, effects PWM control of the switching device Q1 within a switching cycle. This complex control operation is realized by a single control circuit system.
In addition, a direct-current output voltage E04 of 9 V to be supplied to the analog circuit block is obtained from output of the secondary winding (N3+N4) in the power supply circuit, and also a direct-current output voltage E05 of 5 V to be supplied to the digital circuit block is obtained from output of the secondary winding N4.
In this case, the output of the secondary winding (N3+N4) is inputted to a half-wave rectifying and smoothing circuit formed by a rectifier diode D04 and a smoothing capacitor C04 via an inductor L21 (4.7 xcexcH) to reduce power loss. The half-wave rectifying and smoothing circuit first converts the output of the secondary winding (N3+N4) into a direct-current output voltage E07 of 11 V. Then, the direct-current output voltage E04 of 9 V to be outputted to the analog circuit block is obtained from the direct-current output voltage E07.
The output of the secondary winding N4 is inputted to a half-wave rectifying and smoothing circuit formed by a rectifier diode D05 and a smoothing capacitor C05. The half-wave rectifying and smoothing circuit converts the output of the secondary winding N4 into a direct-current output voltage E08 of 6.5 V. Then, the direct-current output voltages E05 (5 V) and E06 (3.3 V) to be outputted to the digital circuit block are obtained from the direct-current output voltage E08.
The direct-current output voltages E04 to E06 to be supplied to the analog and digital circuit blocks need to be made constant so that variations in the voltages fall within a range of xc2x12%.
However, even in the switching power supply circuit employing a complex control method, the level of the direct-current output voltages outputted from the secondary side is varied, though slightly, according to variation in secondary-side load power Po.
For example, as shown in FIG. 10, as the secondary-side load power Po is decreased, the voltage level of the direct-current output voltages E02 (15 V) and E08 (6.5 V) is lowered, though slightly.
Thus, the power supply circuit shown in FIG. 8 is provided with a constant-voltage circuit to obtain a constant direct-current output voltage E04 (9 V) whose variation is within a range of xc2x12% from the direct-current output voltage E07 (11 V) and a constant-voltage circuit to obtain constant direct-current output voltages E05 (5 V) and E06 (3.3 V) whose variation is also within a range of xc2x12% from the direct-current output voltage E08 (6.5 V).
When output current of a constant-voltage circuit is less than 2 A, for example, the constant-voltage circuit is formed by using a three-terminal series regulator IC. When the output current is more than 2 A, the constant-voltage circuit is formed by a step-down type converter using a chopper regulator IC.
In the case of the power supply circuit, the maximum rating of the direct-current output voltage E04 is 9 V/1.5 A, and the output current is less than 2 A. Thus, the constant-voltage circuit for providing the direct-current output voltage E04 is formed by a three-terminal series regulator IC-1 and a smoothing capacitor C041 to thereby provide the direct-current output voltage E04 of 9 V which is made constant within a range of xc2x12%.
The maximum rating of the direct-current output voltage E05 is 5 V/1.5 A, and the output current is less than 2 A. Thus, also in this case, the constant-voltage circuit formed by a three-terminal series regulator IC-2 and a smoothing capacitor C051 provides the direct-current output voltage E05 of 5 V which is made constant within a range of xc2x12%.
On the other hand, the maximum rating of the direct-current output voltage E06 is 3.3 V/3 A, and the output current is more than 2 A. Thus, the direct-current output voltage E08 in this case is inputted via a ferrite-bead inductor FB to a DC-DC converter 11 formed by a PWM control type step-down chopper circuit. The DC-DC converter 11 provides the direct-current output voltage E06 (3.3 Vxc2x10.07 V) which is made constant within a range of xc2x12%.
The DC-DC converter 11 is formed by a chopper regulator IC-3, a flywheel diode D11, and an inductor L22 (20 xcexcH). The DC-DC converter 11 controls its switching operation by feeding back an output voltage outputted via the inductor L22 to the chopper regulator IC to thereby render the level of the output voltage constant.
However, the DC-DC converter 11 exhibits a rectangular waveform in the switching operation, thus causing a high level of noise in the switching operation.
Therefore, the switching noise caused in the switching operation is suppressed by the ferrite-bead inductor FB provided in a stage preceding the chopper regulator IC-3 and a ceramic capacitor Cn provided in a stage succeeding the chopper regulator IC-3.
The direct-current output voltage of the DC-DC converter 11 includes a harmonic ripple voltage component. Therefore, a pi filter circuit 12 formed by electrolytic capacitors C061 and C062 and an inductor L23 (3.3 xcexcH) is provided in the output voltage line to eliminate the high-frequency ripple voltage component.
FIGS. 9A to 9L show operating waveforms of the power supply circuit shown in FIG. 8.
FIGS. 9A to 9F show operating waveforms under conditions where the direct-current output voltages E04 to E06 are made constant so that variations in the voltages fall within a range of xc2x12%, and a total load power of the direct-current output voltages E01 to E06 is 200 W. FIGS. 9G to 9L show operating waveforms under conditions where a total load power of the direct-current output voltages E01 to E06 is 100 W.
When the total load power is 200 W, the switching frequency of the switching device Q1 is controlled to be 71.4 kHz, for example, and the on/off period TON/TOFF of the switching device Q1 is 10 xcexcs/4 xcexcs.
A resonance voltage V1 generated across the primary-side parallel resonant capacitor Cr by the on/off operation of the switching device Q1 is as shown in FIG. 9A, and forms a sinusoidal pulse waveform during the period TOFF during which the switching device Q1 is turned off.
In the meantime, a collector current ICP as shown in FIG. 9B flows through the switching device Q1 .
At the turn-on of the switching device Q1 , a damper current (negative direction) flows through the clamp diode DD1 and the base and collector of the switching device Q1. The damper current period (0.5 xcexcs) during which the damper current flows is a ZVS (Zero Volt Switching) region, and the switching device Q1 is turned on in the ZVS region.
As a result of such switching operation, a voltage V2 generated across the secondary-side parallel resonant capacitor C2 provided on the secondary side of the isolating converter transformer PIT has a resonance waveform as shown in FIG. 9C.
A voltage V3 generated across the secondary winding (N3 +N4) has a resonance waveform as shown in FIG. 9D. An output current I3 as shown in FIG. 9E flows from the secondary winding (N3+N4).
A voltage V5 generated across the secondary winding N5 has a resonance waveform as shown in FIG. 9F.
When the total load power is 100 W, the switching frequency of the switching device Q1 is controlled to be 100 kHz, for example, and the on/off period TON/TOFF of the switching device Q1 is 6 xcexcs/4 xcexcs. In this case, a resonance voltage V1 as shown in FIG. 9G is generated across the primary-side parallel resonant capacitor Cr, and a collector current ICP as shown in FIG. 9H flows through the switching device Q1.
Also in this case, as a result of the switching operation of the switching device Q1, a voltage V2 generated across the secondary-side parallel resonant capacitor C2 has a resonance waveform as shown in FIG. 9I. A voltage V3 generated across the secondary winding (N3+N4) has a resonance waveform as shown in FIG. 9J. A current I3 as shown in FIG. 9K flows from the ending point of the secondary winding N3.
Similarly, a voltage V5 generated across the secondary winding N5 has a resonance waveform as shown in FIG. 9L.
The power supply circuit shown in FIG. 8 has the three-terminal series regulators IC-1 and IC-2 and the DC-DC converter formed with the chopper regulator IC-3 as the constant-voltage circuits for providing constant direct-current output voltages E04 to E06 whose variations are controlled to within a range of xc2x12%. The regulators IC-1 and IC-2 and the DC-DC converter 11 cause power loss.
For example, a power loss of about 3 W occurs in the three-terminal series regulator IC-1 for providing the direct-current output voltage E04. A power loss of about 2.3 W occurs in the three-terminal series regulator IC-2 for providing the direct-current output voltage E05.
Since the DC-DC power conversion efficiency of the DC-DC converter 11 for providing the direct-current output voltage E06 is about 90%, a power loss of about 1.2 W occurs in the DC-DC converter 11.
Hence, when supplying the direct-current output voltages E04 to E06, the power supply circuit shown in FIG. 8 causes a total power loss of about 6.5 W.
In addition, radiators need to be attached to the three-terminal series regulators IC-1 and IC-2, and also the DC-DC converter 11 needs to be provided with the ferrite-bead inductor FB and the ceramic capacitor Cn as components for suppressing the switching noise caused by the switching operation. Thus, the power supply circuit shown in FIG. 8 has a disadvantage of its parts cost being increased with the increase in the number of parts.
Accordingly, in view of the above problems, a switching power supply circuit according to the present invention is comprised as follows.
To achieve the above object, according to a first aspect of the present invention, there is provided a switching power supply circuit, including: a switching means including a switching device for intermittently outputting a direct-current input voltage inputted thereto; an isolating converter transformer including a primary winding and at least first and second secondary windings, the isolating converter transformer being adapted to transmit an output, obtained in the primary winding, of the switching means to the secondary windings and to have a desired degree of coupling to loosely couple the primary winding and the secondary windings to each other; a primary-side parallel resonant circuit formed by the primary winding and a primary-side parallel resonant capacitor, the resonant circuit being provided for converting operation of the switching means into voltage resonance type operation; a secondary-side resonant circuit formed by connecting a secondary-side resonant capacitor to the first secondary winding; a first direct-current output voltage generating means formed by including the secondary-side parallel resonant circuit and adapted to supply a first direct-current output voltage by performing rectifying operation on an alternating voltage obtained from the first secondary winding; a second direct-current output voltage generating means provided with a rectifier circuit for performing rectifying operation on an alternating voltage obtained from the second secondary winding and adapted to supply a second direct-current output voltage; and a constant-voltage control means including a capacitor disposed between a secondary-side reference ground and an anode of a rectifier diode forming the rectifier circuit provided for supplying the second direct-current output voltage; and an active clamp circuit formed by connecting at least a clamp capacitor and an auxiliary switching device in series and disposed in parallel with the capacitor, the constant-voltage control means being adapted to effect constant-voltage control on the second direct-current output voltage by controlling a conduction angle of the auxiliary switching device according to a level of the second direct-current output voltage.
According to a second aspect of the present invention, there is provided a switching power supply circuit, including: a switching means including a switching device for intermittently outputting a direct-current input voltage inputted thereto; an isolating converter transformer including a primary winding and at least first and second secondary windings, the isolating converter transformer being adapted to transmit an output, obtained in the primary winding, of the switching means to the first and second secondary windings and to have a desired degree of coupling to loosely couple the primary winding and the first and second secondary windings to each other; a primary-side parallel resonant circuit formed by the primary winding and a primary-side parallel resonant capacitor, the resonant circuit being provided for converting operation of the switching means into voltage resonance type operation; a secondary-side resonant circuit formed by connecting a secondary-side resonant capacitor to the first secondary winding; a first direct-current output voltage generating means formed by including the secondary-side parallel resonant circuit and adapted to supply a first direct-current output voltage by performing rectifying operation on an alternating voltage obtained from the first secondary winding; a second direct-current output voltage generating means provided with a rectifier circuit for performing rectifying operation on an alternating voltage obtained from the second secondary winding and adapted to supply a second direct-current output voltage; a third direct-current output voltage generating means provided with a rectifier circuit for branching and rectifying an alternating voltage obtained from the second secondary winding and adapted to supply at least a third direct-current output voltage; and a constant-voltage control means including a capacitor disposed between a secondary-side reference ground and an anode of a rectifier diode forming the rectifier circuit provided for supplying the third direct-current output voltage; and an active clamp circuit formed by connecting at least a clamp capacitor and an auxiliary switching device in series and disposed in parallel with the capacitor, the constant-voltage control means being adapted to effect constant-voltage control on the third direct-current output voltage by controlling a conduction angle of the auxiliary switching device according to a level of the third direct-current output voltage.